May 26, 2005
(PLANETQUEST) -- An international collaboration of astronomers, including two well-equipped amateurs in New Zealand, has made the second discovery of an extrasolar planet using gravitational microlensing. Both planets discovered by this means have more mass than Jupiter, but the technique is sensitive enough to identify Earth-size planets as well, according to the discovery paper submitted to Astrophysical Journal Letters.
"Now this method has revealed two planets, showing that the first discovery was not a fluke," said Dr. Avishay Gal-Yam, who leads the California Institute of Technology's contribution to the collaboration.
The newly discovered planet is thought to be about 15,000 light years (90 thousand trillion miles) from Earth, some 2,000 light years closer than the first planet discovered by this method, about one year ago. Both are among the most distant planets ever found.
Predicted by Einstein in 1936, gravitational microlensing is a phenomenon that occurs when a massive object such as a star passes between Earth and a more distant star. The gravitational field of the closer star acts as a lens, magnifying the light of the background star. If a planet orbits the intermediate "lens star," the magnified brightness varies in a characteristic way, enabling scientists to infer the planet's mass and its orbital distance at that moment.
The magnifying lens effect is brief, lasting only as long as Earth and the two stars are sufficiently aligned. In this case, the brightening from the closer star spanned several weeks, and the variation caused by its planet lasted only a few days.
Because of Earth's rotation, tracking the brightening and dimming of the magnified light -- and any telltale variation -- requires tag-team observation around the world. Even advanced amateurs with the right equipment can play an important part, as did the two New Zealanders who were recruited for this effort by the Microlensing Follow-Up Network, one of the groups responsible for the discovery. Grant Christie of Auckland used a 14-inch telescope, while Pakuranga's Jennie McCormick observed with a 10-inch telescope.
"It was truly a collaborative effort," Dr. Gal-Yam observed, "and a nice example of how people around the world can work together."
Gravitational microlensing cannot be used for direct observation of a planet, only to infer the planet's presence by its effect on starlight that happens to pass by en route to Earth. So it can't search a planet's spectral signatures for signs of life. And it's a farsighted technique, well-suited to discovering solar systems halfway to the center of the galaxy, but unable to observe those in our own backyard.
It can, however, help to lay the groundwork for future missions that are expected to examine Earth-size planets within a few hundred light-years of us, like Terrestrial Planet Finder (TPF) and SIM PlanetQuest. "We can start gathering statistics about planets, how common they are, right now," Dr. Gal-Yam noted. "By the time TPF flies, we would like to know how many solar-size stars have Earth-size planets around them. It could affect how you would use TPF."
